Large commercial irrigation systems such as those used on golf courses or croplands use sprinklers, sensors or other components which are normally powered from 24 V AC power lines that can be several miles long and can serve many hundreds of components. Various schemes have been proposed for powering and controlling the components of such a system with just two wires. For example, U.S. Pat. No. 3,521,130 to Davis et al., U.S. Pat. No. 3,723,827 to Griswold et al., and U.S. Pat. No. 4,241,375 to Ruggles disclose systems in which sprinklers along a cable are turned on in sequence by momentarily interrupting the power or transmitting an advance signal from time to time.
A problem with this approach is that it does not allow the operator to freely turn on or off any selected sprinkler or set of sprinklers at different times. This problem is usually resolved by providing separate controllers in the field to operate groups of sprinklers in accordance with a program stored in them, or transmitted to them by radio or other means. Alternatively, it has been proposed, as for example in U.S. Pat. No. 3,578,245 to Brock, to operate individual sprinkler sets from a central location by superimposing a frequency-modulated signal or DC pulses onto the 24 V AC power line. All of these approaches are expensive, and the latter may cause electrolysis problems that can damage the system in the long run.
Finally, a system with hundreds of sprinklers stretched out over miles using conventional electric water valves requires expensive heavy wiring to accommodate the hold-open current drawn by a large number of valves that may be watering simultaneously.
It is therefore desirable to provide an irrigation system in which individual components connected to a two-wire cable can be turned on and off (or, in the case of a sensor component, read) from a central location at minimal cost, with a minimal expenditure of electrical power, and without causing any significant electrolysis problems in the system. It is also desirable to have the ability in such a system to monitor the successful execution of the on-off command, or to return data to the central location, without additional apparatus.